The focal point of reflector antennas of electromagnetic energy is of considerable interest and, in many cases, is sufficient for locating the antenna feed when the focal axis is of no particular concern. However, knowledge of the focal axis is necessary for controlling squint which occurs when the feed is located off the focal axis or for defocusing the feed by movement of the feed back and forth along the focal axis for varying the pattern beamwidth. Both of these concepts are well known and have been used extensively in the prior art.
Focal axis determination has not presented a problem in the past where traditional symmetrical reflectors were utilized because there the focal axis could easily be established by passing a line through the vertex of the antenna perpendicular to the aperture plane defined by the physical rim. Such is not the case, however, for offset antennas currently being used in spacecraft and other ground station type installations due to the fact that the physical rim of the offset antenna is not generally congruent with the aperture plane but is, in fact, elliptical when the aperture plane is circular. Accordingly, a need has arisen for accurately determining the focal axis of an offset reflector which is both simple and can be carried out quickly, thus providing a predictability of the result of feed movement about the focal axis and focal point to control not only squint, but also beamwidth.